Resonance shifting

ABSTRACT

A piezoelectric vibrator comprising: a thin rectangular piezoelectric plate formed of a material having a Young&#39;s modulus having two short edge surfaces and two long edge surfaces and two large planar face surfaces which plate has transverse resonant vibration modes parallel to its short edges and longitudinal resonant vibration modes parallel to its long edges and is formed with at least one cavity; and at least one electrode on each of the planar surfaces that is electrifiable to excite at least one vibration mode of the plate, wherein the at least one cavity is not filled with a material having a Young&#39;s modulus substantially equal to the Young&#39;s modulus of the material from which the plate is formed, such that the presence of the at least one cavity shifts a resonant frequency of at least one vibration mode of the plate with respect to the resonant frequency that characterizes the at least one vibration mode in the absence of the at least one cavity.

RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 10/204,809, filed on Dec. 12, 2002, which is a U.S. nationalfiling of PCT/IL00/00116, filed Feb. 24, 2000.

FIELD OF THE INVENTION

The invention relates to piezoelectric motors and in particular tomethods for shifting the frequencies of resonant vibrations ofpiezoelectric motors.

BACKGROUND OF THE INVENTION

A piezoelectric motor uses a piezoelectric vibrator to transduceelectrical energy into kinetic energy that the motor transmits to amoveable body to which the motor is coupled. The motor is generallycoupled to a body that it moves by resiliently pressing the motor to thebody so that a surface region, hereinafter referred to as a“motor-coupling-surface”, of its piezoelectric vibrator contacts asurface, hereinafter referred to as a “body-coupling-surface”, of thebody. Electrodes comprised in the motor are electrified (generally withan AC voltage) to excite vibrations in the vibrator that cause themotor-coupling-surface to vibrate. Motion is transmitted from thevibrating motor-coupling-surface to move the body by frictional forcesbetween the motor-coupling-surface and the body-coupling-surface.

In general, two orthogonal resonant vibration modes of a piezoelectricvibrator are simultaneously excited to generate vibrations in themotor-coupling-surface that are suitable for transmitting motion to themoveable element. A first resonant vibration mode, hereinafter referredto as a “perpendicular vibration mode”, moves the motor-coupling-surfaceback and forth in a direction perpendicular to thebody-coupling-surface. A second resonant vibration mode, hereinafterreferred to as a “parallel vibration mode”, of the vibrator moves themotor-coupling-surface back and forth parallel to thebody-coupling-surface. As a result of the perpendicular back and forthmotion of the motor-coupling-surface generated by the perpendicularresonant vibration, the two coupling surfaces are alternately coupledand uncoupled during each vibration cycle of the perpendicular vibrationmode. During times when the motor-coupling surface andbody-coupling-surfaces are in contact, motion of themotor-coupling-surface parallel to the body-coupling-surface generatedby the parallel resonant vibration mode transmits motion to the body.

To efficiently transmit motion to a moveable body, the perpendicular andparallel vibration modes of a piezoelectric motor must generally haveexcitation curves as a function of frequency that overlap substantially.As a result of the overlap, electrodes in the piezoelectric motor can beelectrified to excite the vibration modes with an AC driving voltagehaving a frequency, hereinafter referred to as a “driving frequency”, atwhich energy is simultaneously coupled efficiently to both vibrationmodes. In addition, when excited, the perpendicular and parallelvibration modes should optimally have a phase difference, hereinafterreferred to as a “mode phase difference”, close to 90°. If the modephase difference is substantially different from 90°, the perpendicularand parallel vibration modes are not properly synchronized andefficiency with which motion of the motor-coupling-surface transmitsmotion to the body is reduced.

For a piezoelectric vibrator, density and Young's modulus of thepiezoelectric material from which the vibrator is formed and dimensionsof the vibrator determine resonant frequencies of vibration modes of thevibrator. For a given piezoelectric material, characterized by a givenYoung's modulus and density, the dimensions of the vibrator determinethe resonant frequencies. In addition the dimensions determine otheroperational characteristics of the vibrator. For example, the amplitudeof motion of a motor-coupling-surface of the vibrator and the amount ofpower that the vibrator can provide for moving a body are functions ofdimensions of the vibrator as well as the magnitude of an appliedexcitation voltage. Maximum strain in the body of the vibrator, for agiven excitation voltage, is also a function of dimensions of thevibrator.

As a result of the many operational parameters of a piezoelectricvibrator that are functions of the vibrator's dimensions, it isgenerally not possible to determine dimensions of a vibrator thatoptimize all operational characteristics of the vibrator. In particular,it is not always possible or practical to determine dimensions of avibrator so that excitation curves of a perpendicular and a parallelvibration mode overlap at a frequency at which both vibration modes canbe efficiently excited with a 90° mode phase difference. It is thereforeadvantageous to have a method for shifting resonant frequencies of avibrator without having to substantially change dimensions of thevibrator. Such a method could be useable to shift resonant frequenciesof a perpendicular and parallel vibration mode of a vibrator to adjustoverlap of their excitation curves and improve efficiency with which thevibration modes can be simultaneously excited with a mode phasedifference close to 90°.

In some situations, instead of requiring two orthogonal vibration modesto impart motion to a moveable body, a piezoelectric motor is requiredto provide one-dimensional motion of a motor-coupling-surface. For suchmotion, optimum efficiency of operation of the motor is generallyachieved when only a single vibration mode is excited in the motor'svibrator. For a vibrator having dimensions that provide some desiredcharacteristics of the required motion more than one vibration mode maybe excited at a driving frequency at which the motor is to be operated.In such cases it is desirable to have a method for diverging resonantfrequencies of vibration modes that are simultaneously excited withoutsubstantially changing dimensions of the vibrator so as to separatetheir excitation curves and enable excitation of substantially only thesingle vibration mode of the vibrator.

PCT Application PCT/IL99/00576 entitled “Piezoelectric Motors And MotorDriving Configurations”, the disclosure of which is incorporated hereinby reference, describes situations for which excitation of a singlevibration mode is advantageous. In the application, shavers aredescribed that comprise a piezoelectric motor that is used to excitevibrations in cutting blades comprised in the shavers. A one-dimensionalmotion generated in a motor-coupling-surface of the piezoelectric motoris used to excite the vibrations.

Another situation for which it is sometimes desirable to divergeresonant vibration modes of a piezoelectric vibrator occurs, forexample, when it is desired to separately control perpendicular andparallel vibration modes of a piezoelectric vibrator. Optimum control ofthe vibration modes is obtained when the excitation curves of thevibration modes are sufficiently separated so that energy can be coupledto either one of the vibration modes without substantially couplingenergy to the other of the vibration modes. Examples of situations forwhich it is advantageous to excite perpendicular and parallel vibrationmodes of a vibrator separately, and methods for exciting the vibrationmodes, are described in PCT Application PCT/IL99/00288, entitled“Multilayer Piezoelectric Motor”, the disclosure of which isincorporated herein by reference.

It is also desirable to be able to shift a resonant frequency of avibration mode of a vibrator in order to reduce a difference that thefrequency may have from a desired frequency that is caused byinaccuracies in the process by which the vibrator is manufactured. Forexample, assume that in a manufacturing process of a vibrator in theshape of a relatively thin rectangular plate having large face surfacesand narrow long and short edge surfaces, the length of the vibrator isheld to a tolerance of 1%. Assume further that a desired resonantfrequency of a vibration mode of the vibrator for which mass points ofthe vibrator vibrate parallel to the length of the vibrator is, by wayof example 50 kHz. Since the length of the vibrator may vary by as muchas 1%, the resonant frequency, which is proportional to the inverse ofthe length, may vary by as much as 0.5 kHz from 50 kHz. It is desirableto be able to shift the resonant frequency after manufacture of thevibrator to compensate for the variance.

Thin rectangular piezoelectric vibrators are described in U.S. Pat. No.5,616,980 to Zumeris, the disclosure of which is incorporated herein byreference. Various methods are used to mount these motors in machinesand apparatus in which they are used. One of the mounting methods used,which is described in the patent comprises forming mounting holes thatpass through the body of the vibrator in directions perpendicular to thevibrator's large face surfaces. The vibrator is held in place in themachine or apparatus by mounting pins anchored to the machine orapparatus that pass through the mounting holes. Spaces between themounting holes and the mounting pins are preferably filled with aflexible material. As described in the patent, characteristics of thefiller material are preferably chosen to match the acoustic velocity ofthe filler to the acoustic velocity of the piezoelectric material fromwhich the vibrator is formed. Matching the acoustic velocities reducesthe amount by which the holes and the pins disturb resonant frequenciesof the vibrator.

SUMMARY OF THE INVENTION

An aspect of some preferred embodiments of the present invention relatesto providing a method for shifting resonant frequencies of vibrationmodes of a piezoelectric vibrator without substantially changing overalldimensions of the piezoelectric vibrator.

The inventors have found that by introducing perturbations in thestructure of a piezoelectric vibrator that do not substantially changethe overall dimensions of the vibrator resonant frequencies of thevibrator can be shifted.

An aspect of some preferred embodiments of the present invention relatesto perturbing the structure of a piezoelectric vibrator by forming holesin the body of the vibrator to shift resonant frequencies of vibrationmodes of the vibrator.

The inventors have found that resonant frequencies of vibration modes ofa piezoelectric vibrator can be shifted by forming at least one hole inthe vibrator. The magnitude and direction of a shift in a resonantfrequency caused by the at least one hole is generally dependent on thenumber and size of holes and their spatial location relative to nodesand antinodes of the vibration mode.

The presence of a hole in a piezoelectric vibrator decreases the averagevalues of the density and Young's modulus of the piezoelectric materialfrom which the vibrator is formed in the vicinity of the hole or groove.A decrease in Young's modulus decreases the resonant frequencies ofvibration modes of the vibrator. A decrease in density increases theresonant frequencies of vibration modes of the vibrator. A hole in avibrator therefore affects the resonant frequencies of the vibrator inconflicting manners. On one hand a hole tends to decrease the resonantfrequencies of the vibration modes by decreasing (locally) Young'smodulus. On the other hand the same hole tends to increase the resonantfrequencies by decreasing (locally) the density of the vibrator.

However, the amount of the decrease or increase in the resonantfrequency of a vibration mode of the vibrator is dependent upon thelocation of the hole. If the hole is located in a region of the vibratorwhich for a particular vibration mode is a region of maximum strain, theeffect of the hole on Young's modulus tends to dominate the effect ofthe hole on density in shifting the resonant frequency. As a result, theresonant frequency will generally decrease. If on the other hand, thehole is located in a region of minimum strain of the material of thevibrator, the effect of the hole on density tends to dominate the effectof the hole on Young's modulus in shifting the resonant frequency. As aresult, the resonant frequency will generally increase. Regions ofmaximum material strain are located at and near to nodes of thevibration mode while regions of relatively little strain are located atand near antinodes of the vibration mode.

An aspect of some preferred embodiments of the present invention relatesto perturbing the structure of a piezoelectric vibrator by forming atleast one groove in a surface region of the vibrator to shift resonantfrequencies of vibration modes of the vibrator.

The inventors have found that, as in the case of a hole in a vibrator, agroove on the surface of a vibrator generally affects resonantfrequencies of the vibrator by affecting the density and Young's modulusof material from which the vibrator is formed in the vicinity of thegroove. A groove also affects the resonant frequencies by perturbing anexternal dimension of the vibrator on which the resonant frequenciesdepend.

In some preferred embodiments of the present invention, at least onegroove is formed on the surface of a vibrator to introduce aperturbation in a dimension of the vibrator on which the resonantfrequency of a vibration mode of the vibrator depends. The perturbationeffectively shortens the dimension. If the effect of the perturbationdominates Young's modulus and density effects on the resonant frequency,the frequency increases or decreases in response to the shortening ofthe dimension in accordance with the form of the frequency's dependenceon the dimension. The magnitude and direction of a shift in a resonantfrequency caused by the at least one groove is generally dependent onthe number and size of the at least one groove and the location of theat least one groove on the surface of the vibrator.

An aspect of some preferred embodiments of the present invention relatesto shifting resonant frequencies of a piezoelectric vibrator tocompensate for variances of the resonant frequencies of the vibratorfrom desired frequencies. These variances can arise, for example, frominaccuracies in the manufacturing process of the vibrator that result indimensions of the vibrator differing from desired dimensions or fromvariances in characteristics of the piezoelectric material from whichthe vibrator is formed. The magnitudes of the variances can be reducedby shifting the frequencies in accordance with a preferred embodiment ofthe present invention, by forming holes and/or grooves in the vibrator.

An aspect of some preferred embodiments of the present invention relatesto shifting a resonant frequency of a vibration mode of piezoelectricvibrator from a first desired resonant frequency to a second desiredfrequency.

A vibrator manufactured to operate at a first frequency can be adaptedto applications requiring that the vibrator operate at a secondfrequency by forming at least one hole or at least one groove in thevibrator, in accordance with a preferred embodiment of the presentinvention. As a result a manufacturing process used to produce thevibrator that operates at the first frequency does not have to beretooled or substantially changed to produce vibrators that operate atthe second frequency.

An aspect of some preferred embodiments of the present invention relatesto converging or diverging the frequency ranges of excitation curves ofparallel and perpendicular vibration modes of a vibrator of apiezoelectric motor to improve the efficiency with which the motortransmits motion to a body to which it is coupled. The excitation curvesare converged or diverged so that they overlap at a frequency at whichboth modes can be relatively efficiently excited and with a mode phasedifference close to 90°. The excitation curves are converged or divergedby forming at least one hole or at least one groove in the vibrator toshift the resonant frequency of at least one of the vibration modes soas to converge or diverge thereby the excitation curves.

In a preferred embodiment of the present invention the piezoelectricvibrator is a relatively thin rectangular piezoelectric vibrator havingrelatively large parallel face surfaces and narrow long and short edgesurfaces. Preferably, a first order longitudinal vibration mode of thevibrator and a second order transverse vibration mode of the vibratorare respectively perpendicular and parallel vibration modes that areused to impart motion to a body. Whereas the following discussiongenerally relates to a first order longitudinal vibration mode and asecond order transverse vibration mode, results and features describedfor these vibration modes are applicable to other vibration modes of thevibrator.

In a preferred embodiment of the present invention, the resonantfrequency of the second order transverse mode is less than the resonantfrequency of the first order longitudinal vibration mode. At least onehole perpendicular to the large face surfaces is formed in the body ofthe piezoelectric vibrator to converge the resonant frequencies of thevibration modes and thereby the excitation curves of the vibrationmodes. Preferably, the at least one hole is a through hole. The at leastone hole is preferably formed at a point along a long axis of the motorthat passes through the center of the motor parallel to the long edgesof the piezoelectric motor. Preferably the at least one hole is locatedon the axis at an antinode of the transverse vibration mode. Theantinodes of the second transverse mode are located relatively close tothe single node of the first longitudinal vibration mode. The inventorshave found that the at least one hole causes the resonant frequency ofthe longitudinal vibration mode to decrease while the resonant frequencyof the transverse mode increases or is substantially unaffected by theat least one hole. As a result, the resonant frequencies and theexcitation curves of the two modes converge.

If the at least one hole is located at a nodal point of the transversemode that is removed from the nodal point of the longitudinal vibrationmode, the resonant frequency of the transverse mode decreases while theresonant frequency of the longitudinal mode increases and the resonantfrequencies diverge. If the at least one hole is located at the centerof the vibrator, which is a nodal point for both the transverse andlongitudinal vibration modes, the frequencies of both modes decrease.However, the inventors have found that the decrease in the frequency ofthe longitudinal vibration mode tends to be smaller than that of thetransverse mode and the resonant frequencies diverge.

It should be noted that a localized protuberance on a surface of avibrator generates a shift in a resonant frequency of the vibrator thatis opposite to a shift in the resonant frequency that is generated by ahole in the vibrator at the same location as the protuberance. Forexample, in a thin rectangular vibrator, if a hole on a face surface ofthe vibrator increases or decreases a resonant frequency of a vibrationmode of the vibrator, a protuberance in place of the hole will generallyrespectively decrease or increase the resonant frequency.

In a preferred embodiment of the present invention at least one grooveis formed on a narrow edge surface of the piezoelectric vibrator. Theinventors have found that the at least one groove shifts resonantfrequencies of the vibration modes away from each other and divergesthereby their excitation curves. If the at least one groove is locatedon a long edge surface of the vibrator, the at least one groove causesthe resonant frequency of the transverse mode to decrease but does notsubstantially affect the resonant frequency of the longitudinalvibration mode, and diverges thereby the excitation curves. If the atleast one groove is located on a short edge of the vibrator, the atleast one groove does not substantially affect the resonant frequency ofthe transverse vibration mode but causes the resonant frequency of thelongitudinal vibration mode to increase, and diverges thereby theexcitation curves.

An aspect of some preferred embodiments of the present invention relatesto reducing the occurrence of cracks in the body of a piezoelectricvibrator that are encouraged by the presence of a hole in the body ofthe vibrator or a groove on its surface.

In accordance with a preferred embodiment of the present invention alayer of resilient material is bonded to the surface of a hole or grooveformed in the piezoelectric vibrator. The layer functions as a “crackarrest” layer that reduces fracturing of the vibrator in the vicinity ofthe hole or groove that results from strain of piezoelectric materialfrom which the vibrator is formed. Preferably, the crack arrest materialis an epoxy that bonds strongly to the material of the vibrator.

There is therefore provided in accordance with a preferred embodiment ofthe present invention a piezoelectric vibrator comprising:

a thin rectangular piezoelectric plate formed of a material having aYoung's modulus having two short edge surfaces and two long edgesurfaces and two large planar face surfaces which plate has transverseresonant vibration modes parallel to its short edges and longitudinalresonant vibration modes parallel to its long edges and is formed withat least one cavity; and

at least one electrode on each of the planar surfaces that iselectrifiable to excite at least one vibration mode of the plate,

wherein the at least one cavity is not filled with a material having aYoung's modulus substantially equal to the Young's modulus of thematerial from which the plate is formed, such that the presence of theat least one cavity shifts a resonant frequency of at least onevibration mode of the plate with respect to the resonant frequency thatcharacterizes the at least one vibration mode in the absence of the atleast one cavity.

Preferably, the at least one cavity comprises a hole in a planar surfaceof the plate. In some preferred embodiment of the present invention thehole is located in a neighborhood of an antinode of a vibration mode ofthe plate and the presence of the hole increases the resonant frequencyof the vibration mode. Preferably, wherein the hole is locatedsubstantially at the position of the antinode of the vibration mode. Insome preferred embodiment of the present invention the hole is locatedin a neighborhood of a node of a vibration mode and decreases theresonant frequency of the vibration mode. Preferably, the hole islocated substantially at the position of the node of the vibration mode.

In some preferred embodiment of the present invention the at least onevibration mode includes a first and a second vibration mode and the holeis located so as to shift the resonant frequencies of the first andsecond vibration mode of the plate so that the resonant frequenciesconverge.

In some preferred embodiment of the present invention the at least onevibration mode includes a first and a second vibration mode and the holeis located so as to shift the resonant frequencies of the first andsecond vibration mode of the plate so that the resonant frequenciesdiverge.

In some preferred embodiment of the present invention a same singlefrequency AC voltage cannot simultaneously excite the first vibrationmode and the second vibration mode of the plate in the absence of thehole and the convergence caused by the hole is such that the first andsecond vibration modes are simultaneously excitable by a same singlefrequency AC voltage.

In some preferred embodiment of the present invention, in the absence ofthe hole the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage and the convergencecaused by the hole is such that the efficiency with which the first andsecond vibration modes are simultaneously excitable by a same singlefrequency AC voltage is increased.

In some preferred embodiment of the present invention, a same singlefrequency AC voltage cannot simultaneously excite the first and secondvibration modes in the absence of the hole and the convergence caused bythe hole is such that the first and second vibration modes aresimultaneously excitable by a same single frequency AC voltage.

In some preferred embodiment of the present invention when the first andsecond vibration modes are excited by the single frequency AC voltage, aphase difference between the excited first and second vibration modes issubstantially equal to 90°.

In some preferred embodiment of the present invention in the absence ofthe hole a same single frequency AC voltage cannot be used tosimultaneously excite both vibration modes such that a phase differencebetween the excited vibration modes is substantially equal to 90°, andthe shift caused by the presence of the hole is such that a same singlefrequency AC voltage can be used to simultaneously excite both vibrationmodes with a phase difference between the excited vibration modessubstantially equal to 90°.

In some preferred embodiment of the present invention, in the absence ofthe hole a same single frequency AC voltage can efficiently excite bothvibration modes and the divergence caused by the presence of the hole issuch that a same single frequency AC voltage cannot efficiently exciteboth vibration modes.

In some preferred embodiment of the present invention the hole islocated in a neighborhood of a node of the first vibration mode anddecreases the resonant frequency of the first vibration mode.Preferably, the hole is located substantially at the position of thenode of the first vibration mode.

In some preferred embodiment of the present invention wherein the holeis located in a neighborhood of an antinode of the second vibration modeand increases the resonant frequency of the first vibration mode.Preferably, the hole is located substantially at the position of theantinode of the second vibration mode.

In some preferred embodiment of the present invention the hole islocated in a neighborhood of a node of the first vibration mode and aneighborhood of an antinode of the second vibration mode.

In some preferred embodiment of the present invention the firstvibration mode is a longitudinal vibration mode of the plate. In somepreferred embodiment of the present invention the second vibration modeis a transverse vibration mode of the plate.

In some preferred embodiment of the present invention, the hole is athrough hole.

In some preferred embodiment of the present invention, the at least onecavity comprises a groove on at least one long edge surface of the platethat reduces a resonant frequency of a transverse vibration mode of theplate.

In some preferred embodiment of the present invention, the at least onecavity comprises a groove on at least one short edge surface of theplate that increases a resonant frequency of a longitudinal vibrationmode of the plate.

In some preferred embodiment of the present invention, the at least onevibration mode includes first and second vibration modes and the atleast one cavity comprises at least one groove located on an edgesurface of the plate so as to shift the resonant frequencies of thefirst and second vibration modes so that they converge.

In some preferred embodiment of the present invention, the at least onevibration mode includes first and second vibration modes and the atleast one cavity comprises at least one groove located on an edgesurface of the plate so as to shift the resonant frequencies of thefirst and second vibration modes so that they diverge.

In some preferred embodiment of the present invention, a same singlefrequency AC voltage cannot simultaneously excite the first vibrationmode and the second vibration mode of the plate in the absence of the atleast one groove and the convergence caused by the at least one grooveis such that the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage.

In some preferred embodiment of the present invention, in the absence ofthe hole the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage and the convergencecaused by caused by the at least one groove such that the efficiencywith which the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage is increased.

In some preferred embodiment of the present invention, a same singlefrequency AC voltage cannot simultaneously excite the first and secondvibration modes in the absence of the at least one groove and theconvergence caused by the at least one groove is such that the first andsecond vibration-modes are simultaneously excitable by a same singlefrequency AC voltage.

In some preferred embodiment of the present invention, when the firstand second vibration modes are excited by the single frequency ACvoltage, a phase difference between the excited first and secondvibration modes is substantially equal to 90°.

In some preferred embodiment of the present invention, in the absence ofthe at least one groove a same single frequency AC voltage cannot beused to simultaneously excite both vibration modes such that a phasedifference between the excited vibration modes is substantially equal to90°, and the shift caused by the presence of the at least one groove issuch that a same single frequency AC voltage can be used tosimultaneously excite both vibration modes with a phase differencebetween the excited vibration modes substantially equal to 90°.

In some preferred embodiment of the present invention, the firstvibration mode is a longitudinal vibration mode. Preferably, the atleast one groove comprises a groove on a short edge surface of thevibrator that increases the frequency of the vibration mode.

In some preferred embodiment of the present invention, the secondvibration mode is a transverse vibration mode. Preferably, the at leastone groove comprise a groove on a long edge surface of the vibrator.

In some preferred embodiment of the present invention, the platecomprises at least one protuberance formed on a face surface of theplate that shifts a resonant frequency of at least one vibration mode ofthe plate with respect to the resonant frequency that characterizes theat least one vibration mode in the absence of the at least oneprotuberance.

In some preferred embodiment of the present invention, a layer of anelastic material is bonded to the wall of the cavity. In some preferredembodiment of the present invention, a cavity of the at least one cavityis filled with an elastic material that bonds to the material from whichthe plate is formed and has a Young's modulus different from that of thematerial of the plate.

There is further provided, in accordance with a preferred embodiment ofthe present invention piezoelectric vibrator comprising:

a thin rectangular piezoelectric plate having two short edge surfacesand two long edge surfaces and two large planar face surfaces whichplate has transverse resonant vibration modes parallel to its shortedges and longitudinal resonant vibration modes parallel to its longedges and is formed with at least one localized protuberance on a planarface surface; and

at least one electrode on each of the planar surfaces that iselectrifiable to excite a vibration mode of the plate,

wherein the at least one localized protuberance shifts a resonantfrequency of at least one vibration mode of the plate with respect tothe resonant frequency that characterizes the at least one vibrationmode in the absence of the at least one cavity.

In some preferred embodiment of the present invention, the at least oneprotuberance is located in a neighborhood of an antinode of a vibrationmode of the plate and wherein the presence of the protuberance decreasesthe resonant frequency of the vibration mode. Preferably, theprotuberance is located substantially at the position of the antinode ofthe vibration mode.

In some preferred embodiment of the present invention, a protuberance ofthe at least one protuberance is located in a neighborhood of a node ofa vibration mode and increases the resonant frequency of the vibrationmode. Preferably, the protuberance is located substantially at theposition of the node of the vibration mode.

In some preferred embodiment of the present invention, the at least onevibration mode includes a first and a second vibration mode and whereina protuberance of the at least one protuberance is located so as toshift the resonant frequencies of the first and second vibration mode ofthe plate so that the resonant frequencies converge.

In some preferred embodiment of the present invention, the at least onevibration mode includes a first and a second vibration mode and whereina protuberance of the at least one protuberance is located so as toshift the resonant frequencies of the first and second vibration mode ofthe plate so that the resonant frequencies diverge.

In some preferred embodiment of the present invention, a same singlefrequency AC voltage cannot simultaneously excite the first vibrationmode and the second vibration mode of the plate in the absence of theprotuberance and wherein the convergence caused by the protuberance issuch that the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage.

In some preferred embodiment of the present invention, in the absence ofthe protuberance the first and second vibration modes are simultaneouslyexcitable by a same single frequency AC voltage and wherein theconvergence caused by the protuberance is such that the efficiency withwhich the first and second vibration modes are simultaneously excitableby a same single frequency AC voltage is increased.

In some preferred embodiment of the present invention, a same singlefrequency AC voltage cannot simultaneously excite the first and secondvibration modes in the absence of the protuberance and wherein theconvergence caused by the protuberance is such that the first and secondvibration modes are simultaneously excitable by a same single frequencyAC voltage.

In some preferred embodiment of the present invention, when the firstand second vibration modes are excited by the single frequency ACvoltage, a phase difference between the excited first and secondvibration modes is substantially equal to 90°.

In some preferred embodiment of the present invention, in the absence ofthe protuberance a same single frequency AC, voltage cannot be used tosimultaneously excite both vibration modes such that a phase differencebetween the excited vibration modes is substantially equal to 90°, andthe shift caused by the presence of the protuberance is such that a samesingle frequency AC voltage can be used to simultaneously excite bothvibration modes with a phase difference between the excited vibrationmodes substantially equal to 90°.

In some preferred embodiment of the present invention, the firstvibration mode is a longitudinal vibration mode. In some preferredembodiment of the present invention, the second vibration mode is atransverse vibration mode.

In some preferred embodiment of the present invention, the transversevibration mode is a second order transverse vibration mode of thevibrator. In some preferred embodiment of the present invention, thelongitudinal vibration mode is a first order longitudinal vibrationmode.

There is further provided, in accordance with a preferred embodiment ofthe present invention, a method for reducing a tendency for apiezoelectric vibrator to fracture in a neighborhood of a cavity definedby a cavity wall on a surface of the vibrator or in the body of thevibrator comprising: determining an elastic material that bonds to thematerial from which the piezoelectric vibrator is formed; and bondingthe elastic material to the cavity wall.

In some preferred embodiment of the present invention, bonding theelastic material to the cavity wall comprises forming a layer of theelastic material on the cavity wall.

In some preferred embodiment of the present invention, the cavity is ahole in the body of the vibrator and bonding the elastic material to thecavity wall comprises filling the hole with the elastic material.

BRIEF DESCRIPTION OF FIGURES

The invention will be more clearly understood from the followingdescription of preferred embodiments thereof, read with reference to thefigures attached hereto. In the figures, identical structures, elementsor parts that appear in more than one figure are generally labeled withthe same numeral in all the figures in which they appear. Dimensions ofcomponents and features shown in the figures are chosen for convenienceand clarity of presentation and are not necessarily shown to scale. Thefigures are listed below.

FIG. 1A schematically shows a piezoelectric motor according to priorart;

FIG. 1B is a graph that shows dependence of a ratio between the resonantfrequencies of a first order longitudinal and a second order transversevibration mode of the vibrator of the piezoelectric motor shown in FIG.1A on dimensions of the vibrator, according to prior art;

FIGS. 1C–1E are graphs of excitation curves for the first orderlongitudinal vibration mode and second order transverse vibration modeof the piezoelectric motor shown in FIG. 1A for different dimensions ofthe vibrator, according to prior art;

FIG. 2A schematically shows the piezoelectric motor shown in FIG. 1 withholes formed in the body of its vibrator in accordance with a preferredembodiment of the present invention;

FIG. 2B is a graph of excitation curves of the first order longitudinalvibration mode and second order transverse vibration mode, in accordancewith a preferred embodiment of the present invention;

FIG. 2C schematically shows the piezoelectric motor shown in FIG. 1 withprotuberances formed on the body of its vibrator in accordance with andembodiment of the present invention;

FIG. 3A schematically shows another piezoelectric motor according toprior art;

FIG. 3B is a graph of excitation curves of a first order longitudinalvibration mode and a second order transverse vibration mode of thevibrator of the piezoelectric motor shown in FIG. 3A, in accordance witha preferred embodiment of the present invention;

FIG. 4A schematically shows the piezoelectric motor shown in FIG. 3 withholes formed in its vibrator, in accordance with a preferred embodimentof the present invention;

FIG. 4B is a graph of excitation curves of the first order longitudinalvibration mode and second order transverse vibration mode of thevibrator of the piezoelectric motor shown in FIG. 4A;

FIG. 4C schematically shows the piezoelectric motor shown in FIG. 3 withprotuberances formed on the body of its vibrator in accordance with andembodiment of the present invention;

FIG. 5A schematically shows the piezoelectric motor shown in FIG. 3 withgrooves formed on long edge surfaces of its vibrator, in accordance witha preferred embodiment of the present invention;

FIG. 5B is a graph of excitation curves of the first order longitudinalvibration mode and second order transverse vibration mode of thevibrator of the piezoelectric motor shown in FIG. 5A;

FIG. 6A schematically shows the piezoelectric motor shown in FIG. 3 withgrooves formed on a short edge surface of its vibrator, in accordancewith a preferred embodiment of the present invention and;

FIG. 6B is a graph of excitation curves of the first order longitudinalvibration mode and second order transverse vibration mode of thevibrator of the piezoelectric motor shown in FIG. 6A;

FIGS. 7A–7D schematically show different configurations of holes andgrooves in a piezoelectric motor that are lined or filled with aresilient “crack arrest” material, in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1A schematically shows a piezoelectric motor 20 of a type describedin U.S. Pat. No. 5,453,653 to Zumeris et al, the disclosure of which isincorporated herein by reference. Piezoelectric motor 20 comprises athin rectangular piezoelectric vibrator 22 having long edge surfaces 24and short edge surfaces 26 and large parallel face surfaces 28. Thelength of long edge surfaces 22 and the length of short edge surfaces 26(i.e. the width of vibrator 22) are noted as “L” and “W” respectively. Along axis 30 of piezoelectric motor 20, shown with a dashed line, passesthrough the center of vibrator 22 parallel to long edge surfaces 24.Various surface regions of piezoelectric motor 20 may be used for amotor-coupling-surface and some piezoelectric motors similar topiezoelectric motor 20 have more than one motor-coupling-surface.Generally, a region of a short edge surface 26 or a surface of afriction nub located on a short edge surface 26 functions as amotor-coupling-surface. In FIG. 1A, piezoelectric motor 20 is shown, byway of example, with a friction nub 32 mounted to a short edge 26 of thepiezoelectric motor.

A longitudinal vibration mode of vibrator 22, for which mass points ofvibrator 22 and friction nub 32 vibrate parallel to axis 30, functionsas a perpendicular vibration mode of the vibrator. The longitudinalvibration mode couples vibrator 22 to a body that piezoelectric motor 20moves. Preferably, the longitudinal mode is a first order longitudinalmode. A transverse vibration mode of vibrator 22, for which mass pointsof the vibrator and friction nub 32 vibrate parallel to short edgesurfaces 26, functions as a parallel vibration mode of the vibrator.Motion of friction nub 32 generated by the transverse vibration modetransmits energy to the body that the motor moves when the longitudinalvibration mode couples vibrator 22 to the body. Preferably, thetransverse vibration mode is a second order transverse vibration mode ofvibrator 22.

Directions of the longitudinal and transverse vibrations are indicatedby double arrowhead lines 34 and 36 respectively. The longitudinal andtransverse vibration modes generate elliptical vibratory motion infriction nub 32, which elliptical motion is represented by an ellipse38. Electrodes (not shown) on large face surfaces 28 of vibrator 22 areelectrified with an AC driving voltage to excite the longitudinal andtransverse vibration modes. For a given magnitude AC driving voltage,the eccentricity of ellipse 38 and the direction of its major axis arefunctions of a phase difference between the longitudinal and transversevibration modes.

The relative amplitude of the first order longitudinal vibration mode asa function of position along axis 30 is represented by the distance of acurve 40, shown with a dashed line, above or below axis 30. The distanceof a curve 42 above or below axis 30 represents the relative amplitudeof the second transverse vibration mode as a function of position alongaxis 30. Nodal regions of the longitudinal and transverse vibrationmodes are plane surfaces in vibrator 22 that are parallel to short edgesurfaces 26 and pass through axis 30 at points at which curves 40 and 42respectively intersect axis 30.

To within a scale factor determined by the length L of piezoelectricvibrator 22, the frequencies of the resonant transverse and longitudinalvibration modes of piezoelectric vibrator 22 are substantiallydetermined by a ratio, hereinafter referred to as an “aspect ratio”, ofthe width W to the length L of the vibrator. The transverse resonantfrequencies of piezoelectric vibrator 22 are strongly dependent on theaspect ratio while the longitudinal resonant frequencies are relativelyinsensitive to the aspect ratio. The resonant frequencies scalesubstantially as the inverse of the length of piezoelectric vibrator 22.Methods for calculating values for the frequencies of the resonantlongitudinal and transverse vibration modes of piezoelectric vibrator 22are well known in the art. Such methods may be found, for example, in“Vibration Problems in Engineering” by S. Timoshenko and D. H. Young(co-author of the third edition) or in “Analysis of PiezoelectricMultiple Mode Resonators Vibrating in Longitudinal and Flexural Modes”;H. Jumonji; Electronics and Communications in Japan; Vol 51A, No. 3,1968, pp 35–42, the disclosures of which are incorporated herein byreference.

FIG. 1B is a graph of a ratio of the resonant frequency of the secondtransverse vibration mode of vibrator 22 to the resonant frequency ofthe first longitudinal vibration mode of vibrator 22 as a function ofaspect ratio of the vibrator. The aspect ratio, represented by “α”(α=W/L), is shown along the abscissa and the ratio of the resonantfrequencies, represented by “FR”, is shown along the ordinate. A curve52 shows the values of FR as a function of α for piezoelectric vibrator22. A straight line 54 parallel to the abscissa having ordinate equal toone represents the resonant frequency, normalized to itself, of thefirst longitudinal resonant mode of piezoelectric vibrator 22.

Curve 52 crosses line 54 at aspect ratios α₁ and α₂. For these aspectratios, resonant frequencies of the transverse and longitudinalvibration modes are equal and the excitation curves of the vibrationmodes overlap substantially completely. However, for these aspectratios, the mode phase difference between the transverse andlongitudinal vibration modes is substantially equal to zero. Thevibration modes are substantially in phase. As a result, the vibrationmodes are highly inefficient at transmitting motion to a body to whichpiezoelectric motor 22 is coupled.

FIG. 1C is a graph, for aspect ratio α₁, that shows schematic excitationcurves 58 and 60 for the longitudinal and transverse vibration modesrespectively as functions of frequency ω, i.e. a driving frequency, ofan AC driving voltage used to excite the vibration modes. Excitationcurve 60 is shown with a dashed line and slightly offset from excitationcurve 58 for clarity of presentation. Curves 58 and 60 have a maximum ata frequency ω_(o)(α₁) which is a resonant frequency of both thelongitudinal and transverse vibration modes for aspect ratio α₁Excitation curves 58 and 60 are normalized so that their maxima areequal to one. The height of excitation curves 58 and 60 at any frequencyis therefore equal to the relative efficiency with which energy iscoupled to the longitudinal and transverse vibration modes respectivelycompared to the efficiency with which energy is coupled to the vibrationmodes at the resonant frequency ω_(o)(α₁). An ordinate axis forexcitation curves 58 and 60 is shown at the left of FIG. 1C. The scaleof the abscissa in FIG. 1C is arbitrary.

A curve 62 in FIG. 1C schematically graphs a phase difference,hereinafter referred to as an “excitation phase difference”, between thephase of each of the vibration modes and the phase of the drivingvoltage as a function of ω. (The excitation phase difference isdifferent from the mode phase difference, which is the differencebetween the phases of the vibration modes.) Since the mode phasedifference between the longitudinal and transverse vibration modes issubstantially zero, the excitation phase differences for both modes aresubstantially the same for all frequencies ω. A single curve (curve 62)is therefore sufficient to graph the excitation phase differences forboth vibration modes. The magnitude of an excitation phase difference isshown along an ordinate axis at the right of FIG. 1C. For frequenciesless than ω_(o)(α₁) the phase of the driving voltage lags the phases ofthe vibration modes and the excitation phase difference of bothvibration modes is negative. For frequencies substantially less thanω_(o)(α₁), the excitation phase difference is close to −90°. Forfrequencies greater than ω_(o)(α₁), the phase of the driving voltageleads the vibration modes and the excitation phase difference of bothvibration modes is positive. For frequencies substantially greater thanω_(o)(α₁) the excitation phase difference is close to +90°.

In order to excite the longitudinal and transverse vibration modes witha mode phase difference close to 90°, the resonant frequencies of thelongitudinal and transverse vibration modes have to be diverged. Thiscan be accomplished by choosing an appropriate aspect ratio for vibrator22 in the neighborhood of either aspect ratio α₁ or aspect ratio α₂ thatis greater than or less than aspect ratio α₁ or α₂ respectively.

For aspect ratios in the neighborhood of aspect ratio α₂, the width ofvibrator 22 is relatively large and the amplitude of the transversevibration mode for aspect ratios in this neighborhood is generally toosmall to be useful for transmitting motion to a body to whichpiezoelectric motor 20 is coupled. On the other hand, aspect ratios inthe neighborhood of aspect ratio α₁ generally provide amplitudes ofmotion for both the longitudinal and transverse vibration modes that aresuitable for moving the body.

Let α₃ (shown in FIG. 1B) represent an aspect ratio for vibrator 22 inthe neighborhood of α₁ for which the transverse and longitudinalvibration modes, when excited, have a 90° mode phase difference. Since,for a same excitation voltage used to excite vibrations in piezoelectricvibrator 22, an amplitude of vibration of the transverse vibration modeis larger for a lower aspect ratio than for a higher aspect ratio,preferably α₃ is less than α₁. In FIG. 1B, α₃ is shown by way of examplehaving a value smaller than α₁.

FIG. 1D is a graph for aspect ratio α₃ that shows schematic excitationcurves 58 and 60 for the longitudinal and transverse vibration modes.The resonant frequencies of the longitudinal and transverse vibrationmodes, at which frequencies excitation curves 58 and 60 have maxima, arerepresented by ω_(oL)(α₃) and ω_(oT)(α₃) respectively. Curves 59 and 61associated with excitation curves 58 and 60 respectively, graph theexcitation phase difference as a function of ω for the longitudinal andtransverse vibration modes. Excitation curves 58 and 60 overlapsufficiently so that in a range of frequencies in the neighborhood of adriving frequency ω_(D), both longitudinal and transverse vibrationmodes are relatively efficiently excited. The mode phase difference atω_(D), which is the difference between the ordinates of excitation phasecurves 59 and 61 at ω_(D), is approximately equal to 90°.

For some applications, amplitudes of the transverse vibration mode ofvibrator 22 at aspect ratio α₃ are not large enough and it isadvantageous to choose an aspect ratio, α₄, which is shown in FIG. 1B,for piezoelectric motor 22 that is smaller than α₃. FIG. 1E is a graphof excitation curves 58 and 60 and associated excitation phase curves 59and 61 for aspect ratio α₄. From the curves it is readily seen thatresonant frequencies ω_(oL)(α₄) and ω_(oT)(α₄) are relatively far apartand there is no frequency for which both longitudinal and transversevibration modes can both be efficiently excited simultaneously. Foraspect ratio α₄, piezoelectric motor 22 is practically inoperable.

FIG. 2A schematically shows piezoelectric motor 22 shown in FIG. 1Ahaving an aspect ratio α₄ with holes 64 formed in the body of vibrator22, in accordance with a preferred embodiment of the present invention.Holes 64 are preferably through holes located along axis 30 at antinodesof the (second order) transverse vibration mode of piezoelectricvibrator 22. For the transverse vibration mode therefore, the effect ofholes 64 on density dominates the effect of holes 64 on Young's modulusin shifting resonant frequency ω_(oT)(α₄) of the vibration mode, andω_(oT)(α₄) increases. The antinodes of the transverse vibration mode andtherefore holes 64 are located near to the single nodal point of thelongitudinal vibration mode on axis 30. For the longitudinal vibrationmode, the effect of holes 64 on Young's modulus dominates the effect ofholes 64 on density and resonant frequency ω_(oL)(α₄) decreases.Therefore, as a result of the presence of holes 64, in accordance with apreferred embodiment of the present invention, excitation curves 58 and60, which are normally far apart for aspect ratio α₄, converge andoverlap substantially.

FIG. 2B is a graph that shows excitation curves 58 and 60 and excitationphase curves 59 and 61 for aspect ratio α₄ as functions of frequency ωfor the longitudinal and transverse vibration modes of piezoelectricvibrator 22 formed with holes 64. From excitation curves 58 and 60 it isseen that the convergence of excitation curves 58 and 60 is sufficientso that there is a range of frequencies in a neighborhood of a frequencyω_(D) at which both longitudinal and transverse vibration modes can beefficiently excited. From excitation phase curves 59 and 61 it is seenthat at ω_(D) a mode phase difference for the vibration modes issubstantially equal to 90°. Solid and dashed arrows 65 and 66 indicatedirections in which holes 64 move excitation curves 58 and 60respectively compared to the locations of excitation curves 58 and 60shown in FIG. 1E. As a result of holes 64, piezoelectric motor 20 isoperable with vibrator 22 having aspect ratio α₄.

It should be noted that whereas holes 64 are shown as through holes, insome preferred embodiments of the present invention holes 64 are “blind”holes that do not penetrate completely through vibrator 22. In addition,it should be noted that the location of holes 64 shown in FIG. 2A is byway of example and holes, in accordance with a preferred embodiment ofthe present invention, can be formed in different locations ofpiezoelectric vibrator 22, and such different locations can beadvantageous.

It is noted that were protuberances located at the antinode locations atwhich holes 64 are located, as noted above, the protuberances would havean effect on the longitudinal and transverse frequencies of vibrator 22opposite to that of holes 64. FIG. 2C schematically shows vibrator 22with protuberances 164 in place of holes 64 shown in FIG. 2A.Protuberances 164 would cause excitation curves 58 and 60 shown in FIGS.1A–1C to move away from each other rather than towards each other.

By way of example illustrating the effect of holes 64 on frequencies ofthe longitudinal and transverse vibration modes in vibrator 22 of apiezoelectric motor 20, assume that vibrator 22 is typical of prior artvibrators of its type. Let vibrator 22 have a thickness equal to 3 mm, awidth W=7.5 mm, a length L=30 mm and be formed from a piezoelectricmaterial having a Young's modulus equal to 7.9×10¹⁰ N/m². For thisconfiguration of piezoelectric vibrator 22, a first longitudinalvibration mode of the vibrator has a resonant frequency of about 54 kHzand a second transverse vibration mode of the vibrator has a resonantfrequency equal to 48.7 kHz.

The second transverse vibration mode of piezoelectric vibrator 22 forthe above specifications has an antinode along axis 30 located 10 mmfrom each short edge surface 24. The inventors have found that forming ahole having a diameter of about 2 mm that passes through axis 30 ofmotor 22 at each antinode, shifts the longitudinal frequency from 54 kHzto 51.8 kHz and the transverse frequency from 48.7 kHz to 50.9 kHz.Preferably vibrator 22 is excited by an AC voltage having a frequencyabout 49.9 kHz.

In some situations it is desirable to diverge excitation curves of tworesonant vibration modes of a piezoelectric motor instead of convergethem. Such a situation can occur when it is advantageous to excite onlyone vibration mode in a vibrator.

FIG. 3A shows a piezoelectric motor 25 similar to piezoelectric motor 20shown in FIG. 1A. Piezoelectric motor 25 comprises a vibrator 27 thathas a first order longitudinal vibration mode and a second ordertransverse vibration mode characterized by relatively broad excitationcurves that overlap substantially. FIG. 3B is a graph that showsschematic excitation curves 67 and 69 for the longitudinal andtransverse vibration modes respectively. The resonant frequencies of thelongitudinal and transverse vibration modes are respectively noted asω_(oL) and ω_(oT). Assume that in operation of piezoelectric motor 25 itis desired to excite only the longitudinal vibration mode of vibrator27. From graph 65 it is seen that it is difficult to excite one of thevibration modes without exciting the other. Excitation of one of thevibration modes without the other can be accomplished using speciallydesigned electrodes. However, when piezoelectric motor 25 is coupled toa load, as a result of coupling to the load, the vibration modes can becoupled to each other. As a result the efficiency with which thespecially designed electrodes couple energy to only one of the modes butnot the other is reduced. The inventors have found that by forming atleast one hole in vibrator 27 that is preferably located on axis 30 at anode of the second transverse vibration of the vibrator, excitationcurves 67 and 69 diverge and the amount of their overlap is reduced. Theeffect of holes at nodes of the second transverse vibration mode of avibrator similar to that shown in FIGS. 1–3 is opposite to the effect ofholes located at antinodes of the vibration mode.

FIG. 4A shows piezoelectric motor 25 with two holes 70 formed invibrator 24. Preferably, holes 70 are through holes located along axis30 of vibrator 25. Preferably, one hole 70 is formed at a node of thesecond resonant transverse vibration near to each short edge surface 26of vibrator 25. For the transverse vibration mode the effect of holes 70on Young's modulus dominates the effect of holes 70 on density inshifting resonant frequency ω_(oT), and ω_(oT) decreases. The nodes ofthe transverse vibration mode are located relatively far from the singlenodal point on axis 30 of the longitudinal vibration mode. For thelongitudinal vibration mode the effect of holes 70 on density dominatesthe effect of holes 70 on Young's modulus and resonant frequency ω_(oL)increases. Therefore holes 70 cause excitation curves 67 and 69 shown inFIG. 3B to diverge.

FIG. 4B shows schematic excitation curves 67 and 69 for piezoelectricvibrator 25 formed with holes 70. Solid and dashed arrows 65 and 66indicate directions in which holes 70 move excitation curves 67 and 69respectively compared to the locations of excitation curves 67 and 69shown in FIG. 3B.

Similarly to the case of holes 64, were protuberances located at thenode locations at which holes 70 are located, as noted above, theprotuberances would have an effect on the longitudinal and transversefrequencies of vibrator 24 opposite to that of holes 64. FIG. 4Cschematically shows vibrator 24 with protuberances 170 in place of holes70 shown in FIG. 4A. Protuberances 170 would cause excitation curves 67and 69 shown in FIGS. 3B and 4B to move toward each other rather tanaway from each other.

The inventors have found that a groove on the surface of a vibrator canbe used to shift resonant frequencies of vibration modes of thevibrator. For a vibrator of a type shown in the preceding figures theinventors have found that at least one groove on a short edge surface ora long edge surface of the vibrator shifts resonant frequencies ofvibration modes of the vibrator.

For example, the inventors have found that at least one groove formed ona long edge surface 24 of vibrator 25 shown in FIGS. 3 and 4, inaddition to or instead of holes 70 may be used, in accordance with apreferred embodiment of the present invention, to diverge excitationcurves 67 and 69.

FIG. 5A shows piezoelectric vibrator 25 with a plurality of grooves 74preferably formed on long edge surfaces 24 of piezoelectric vibrator 22,in accordance with a preferred embodiment of the present invention.Preferably, for each groove 74 on one long edge surface 24 there is agroove opposite it on the other long edge 24. Grooves 74 perturb width Wof vibrator 27 and cause the width, and thereby the aspect ratio, ofvibrator 27 to be locally decreased in the neighborhoods of grooves 74.Generally, for most useful ranges of aspect ratio α, the resonantfrequencies of the transverse vibration modes decrease with decreasingaspect ratio. The effect of grooves 74 on width W therefore tends todecrease the resonant frequencies of the transverse vibration modes andcooperates with the effect of grooves 70 on Young's modulus, which alsotends to decrease the resonant frequencies of the transverse mode.Grooves 74 therefore generally tend to decrease the resonant frequenciesof the transverse vibration modes. On the other hand, grooves 74 do notgenerally substantially affect the resonant frequencies of thelongitudinal modes. As a result, grooves 74 diverge excitation curves 67and 69. Preferably, in using grooves 74 to decrease the resonantfrequency of a transverse vibration mode of vibrator 27 the grooves arelocated at or close to nodal regions of the transverse vibration mode onlong edge surfaces 24.

FIG. 5B is a graph that shows schematic excitation curves 67 and 69diverged as a result of grooves 74. Since grooves 74 do notsubstantially affect the resonant frequency of the longitudinalvibration mode, excitation curve 67 in FIG. 5B is located insubstantially the same range of frequencies as excitation curve 67 inFIG. 3B. Dashed arrow 66 indicates the direction in which groove 74moves excitation curve 69 compared to the location of excitation curve69 shown in FIG. 3B.

FIG. 6A shows piezoelectric vibrator 25 with a groove 76 formed on theshort edge surface 26 of the vibrator opposite to friction nub 32, inaccordance with a preferred embodiment of the present invention. Groove76 perturbs the length L of vibrator 27 and “locally” shortens thelength. The longitudinal resonant frequencies are generally inverselyproportional to length L. The effect of groove 76 on L therefore tendsto increase the resonant frequencies of the longitudinal vibration modesand cooperates with the effect of groove 76 on mass density, which alsotends to increase the resonant frequencies of the longitudinal mode. Asa result, groove 76 generally increases the resonant frequencies of thelongitudinal vibration modes of vibrator 27. The inventors have foundthat groove 76 does not substantially affect the resonant frequencies ofthe transverse vibration modes. As a result, groove 76 also divergesexcitation curves 67 and 69.

FIG. 6B is a graph that shows schematic excitation curves 67 and 69diverged as a result of groove 76. Since groove 76 does notsubstantially affect the resonant frequency of the transverse vibrationmode, excitation curve 69 in FIG. 6B is located in substantially thesame range of frequencies as excitation curve 69 in FIG. 3B. Solid arrow65 indicates the direction in which groove 76 move excitation curve 67compared to the location of excitation curve 67 shown in FIG. 3B.

It should be noted that in the above discussion and preceding figures,the resonant frequency of the second order transverse vibration mode islower than the resonant frequency of the first order longitudinalvibration mode (except for the situation shown in FIG. 1 in which theresonant frequencies are equal). This is of course not necessary and inaccordance with preferred embodiments of the present invention, theresonant frequency of the second order transverse vibration mode can begreater than the resonant frequency of the first order longitudinalvibration mode. The effect of at least one hole and at least one groovein diverging or converging resonant frequencies as described above isreversed if the resonant frequency of the transverse vibration mode isgreater than the resonant frequency of the longitudinal mode in thepreceding figures.

In the above description piezoelectric vibrators are shown havingeither, but not both, at least one hole or at least one groove to shiftresonant frequencies of the vibrators, in accordance with a preferredembodiment of the present invention. In some preferred embodiments ofthe present invention a vibrator is formed with both at least one holeand at least one groove to shift resonant frequencies of the vibrator.

Piezoelectric ceramics from which piezoelectric motors are generallyformed are relatively brittle materials that are prone to fracturingthat can result in “catastrophic” failure of the material. For avibrator formed with a hole or a groove, in accordance with a preferredembodiment of the present invention, fracturing of the material of thepiezoelectric motor is generally enhanced in the vicinity of the hole orgroove.

In some preferred embodiments of the present invention, the surface of ahole or groove formed in a piezoelectric vibrator is bonded with a layerof elastic material that functions as a crack arrest layer to reducefracturing of the material of the vibrator in the vicinity of the holeor groove. In some preferred embodiments of the present invention, thehole or groove is filled with an elastic material to reduce fracturing.Preferably, the elastic material is a flexible epoxy. In determining theamount by which a hole or groove shifts a resonant frequency of thevibrator, the effect of Young's modulus and the density of the elasticmaterial used as a filler must be taken into account.

FIG. 7A schematically shows a piezoelectric motor 78, similar topiezoelectric motor 25, having holes 70, in which the walls of the holesare bonded with a layer of an elastic crack arrest material 80 to reducefracturing, in accordance with a preferred embodiment of the presentinvention. FIG. 7B shows motor 78 in which holes 70 are filled with anelastic material 84 to arrest cracking, in accordance with a preferredembodiment of the present invention. FIG. 7C shows motor 78 havinggrooves 74 formed on its long edges 24, which the grooves are lined witha layer of crack arrest material 80, in accordance with a preferredembodiment of the present invention. FIG. 7D shows grooves 74 filledwith an elastic crack arrest material 84, in accordance with a preferredembodiment of the present invention.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofpreferred embodiments thereof that are provided by way of example andare not intended to limit the scope of the invention. The describedpreferred embodiments comprise different features, not all of which arerequired in all embodiments of the invention. Some embodiments of thepresent invention utilize only some of the features or possiblecombinations of the features. Variations of embodiments of the presentinvention that are described and embodiments of the present inventioncomprising different combinations of features noted in the describedembodiments will occur to persons of the art. The scope of the inventionis limited only by the following claims.

1. A piezoelectric vibrator comprising: a thin rectangular piezoelectricplate having two short edge surfaces and two long edge surfaces and twolarge planar face surfaces which plate has transverse resonant vibrationmodes parallel to its short edges and longitudinal resonant vibrationmodes parallel to its long edges and is formed with at least onelocalized protuberance on a planar face surface that is located in aneighborhood of a node of a vibration mode and increases the frequencyof the vibration mode; and at least one electrode on each of the planarsurfaces that is electrifiable to excite a vibration mode of the plate,wherein the at least one localized protuberance shifts a resonantfrequency of at least one vibration mode of the plate with respect tothe resonant frequency that characterizes the at least one vibrationmode in the absence of the at least one protuberance.
 2. A vibratoraccording to claim 1 wherein the protuberance is located substantiallyat the position of the antinode of the vibration mode.
 3. A vibratoraccording to claim 1 wherein the protuberance is located substantiallyat the position of the node of the vibration mode.
 4. A vibratoraccording to claim 1 wherein the at least one vibration mode includes afirst and a second vibration mode and wherein a protuberance of the atleast one protuberance is located so as to shift the resonantfrequencies of the first and second vibration mode of the plate so thatthe resonant frequencies converge.
 5. A vibrator according to claim 1wherein the at least one vibration mode includes a first and a secondvibration mode and wherein a protuberance of the at least oneprotuberance is located so as to shift the resonant frequencies of thefirst and second vibration mode of the plate so that the resonantfrequencies diverge.
 6. A vibrator according to claim 4 wherein a samesingle frequency AC voltage cannot simultaneously excite the firstvibration mode and the second vibration mode of the plate in the absenceof the protuberance and wherein the convergence caused by theprotuberance is such that the first and second vibration modes aresimultaneously excitable by a same single frequency AC voltage.
 7. Avibrator according to claim 4 wherein the convergence caused by theprotuberance is such that the efficiency with which the first and secondvibration modes are simultaneously excitable by a same single frequencyAC voltage is increased.
 8. A vibrator according to claim 4 wherein asame single frequency AC voltage cannot simultaneously excite the firstand second vibration modes in the absence of the protuberance andwherein the convergence caused by the protuberance is such that thefirst and second vibration modes are simultaneously excitable by a samesingle frequency AC voltage.
 9. A vibrator according to claim 7 whereinwhen the first and second vibration modes are excited by the singlefrequency AC voltage, a phase difference between the excited first andsecond vibration modes is substantially equal to 90°.
 10. A vibratoraccording to claim 4 wherein in the absence of the protuberance a samesingle frequency AC voltage cannot be used to simultaneously excite bothvibration modes such that a phase difference between the excitedvibration modes is substantially equal to 90°, and the shift caused bythe presence of the protuberance is such that a same single frequency ACvoltage can be used to simultaneously excite both vibration modes with aphase difference between the excited vibration modes substantially equalto 90°.
 11. A vibrator according to claim 4 wherein the first vibrationmode is a longitudinal vibration mode.
 12. A vibrator according to claim4 wherein the second vibration mode is a transverse vibration mode. 13.A vibrator according to claim 4 wherein the transverse vibration mode isa second order transverse vibration mode of the vibrator.
 14. A vibratoraccording to claim 1 wherein the longitudinal vibration mode is a firstorder longitudinal vibration mode.
 15. A vibrator according to claim 5wherein the first vibration mode is a longitudinal vibration mode.
 16. Avibrator according to claim 5 wherein the second vibration mode is atransverse vibration mode.
 17. A piezoelectric vibrator comprising: athin rectangular piezoelectric plate having two short edge surfaces andtwo long edge surfaces and two large planar face surfaces which platehas transverse resonant vibration modes parallel to its short edges andlongitudinal resonant vibration modes parallel to its long edges and isformed with at least one localized protuberance on a planar face surfacethat is located in a neighborhood of an antinode of a vibration mode andincreases the frequency of the vibration mode; and at least oneelectrode on each of the planar surfaces that is electrifiable to excitea vibration mode of the plate, wherein the at least one localizedprotuberance shifts a resonant frequency of at least one vibration modeof the plate with respect to the resonant frequency that characterizesthe at least one vibration mode in the absence of the at least oneprotuberance.